This invention relates to vehicle detectors used to detect the presence or absence of a motor vehicle in an inductive loop embedded in a roadbed.
Vehicle detectors have been used for a substantial period of time to generate information specifying the presence or absence of a vehicle at a particular location sometimes termed a detection zone. Such detectors have been used at intersections, for example, to supply information used by an associated traffic control unit to control the operation of the traffic signal heads, and have also been used to supply control information used in conjunction with automatic entrance and exit gates in parking lots, garages and buildings. A widely used type of vehicle detector employs the principle of period shift measurement in order to determine the presence or absence of a vehicle in or adjacent the inductive loop mounted on or in a roadway. In such systems, a first oscillator, which typically operates in the range from about 10 to about 120 Khz is used to produce a periodic signal in a vehicle detector loop. A second oscillator operating at a much higher frequency is commonly used to generate a sample count signal over a fixed number of loop cycles. The relatively high frequency count signal is typically used to increment a counter, which stores a number corresponding to the sample count at the end of the fixed number of loop cycles. This sample count is compared with a reference count stored in another counter and representative of a previous count in order to determine whether a vehicle has entered or departed the region of the loop in the time period between the previous sample count and the present sample count.
The initial reference value is obtained from one or more initial sample counts and stored in a reference counter. Thereafter, successive sample counts are obtained on a periodic basis, and compared with the reference count. If the two values are essentially equal, the condition of the loop remains unchanged, i.e., a vehicle has not entered or departed the loop. However, if the two numbers differ by at least a threshold amount in a first direction (termed the Call direction), the condition of the loop has changed and may signify that a vehicle has entered the loop. More specifically, in a system in which the sample count has decreased and the sample count has a numerical value less than the reference count by at least a threshold magnitude, this change signifies that the period of the loop signal has decreased (since fewer counts were accumulated during the fixed number of loop cycles), which in turn indicates that the frequency of the loop signal has increased, usually due to the presence of a vehicle in or near the loop. When these conditions exist, the vehicle detector generates a signal termed a Call signal indicating the presence of a vehicle in the loop.
Correspondingly, if the difference between a sample count and the reference count is greater than a second threshold amount, this condition indicates that a vehicle which was formerly located in or near the loop has left the detection zone. When this condition occurs, a previously generated Call signal is dropped.
Call signals are used in a wide variety of applications, including vehicle counting along a roadway or through a parking entrance or exit, vehicle speed between preselected points along a roadway, vehicle presence at an intersection controlled by a traffic control light system, or in a parking stall, and numerous other applications.
In addition to the basic function of generating and dropping a Call signal, existing vehicle detectors incorporate other features, some of which are selectable by a technician. For example, in order to avoid the false generation of a Call signal, some existing vehicle detectors are provided with a Call delay feature which requires that the vehicle be present in the detection zone for a minimum time period before the vehicle detector is permitted to generate a Call signal (in order to screen out false Calls). Further, some vehicle detectors incorporate a maximum presence function which permits a Call signal to persist for only a maximum period of time, after which the Call signal is automatically dropped (typically to prevent a vehicle stalled over a loop from continuing to generate a Call signal). Still other vehicle detectors incorporate an end of green function which requires the detector to automatically reset after the green traffic signal, which controls the lane in which the loop associated with the vehicle detector is located, terminates. Some vehicle detectors are provided with an extension time feature which extends the Call signal for a period of time after a vehicle leaves the associated loop (typically in order to permit ample minimum time for a vehicle to clear an intersection). Some vehicle detectors are also provided with a presence/pulse selection feature, which causes the vehicle detector to generate one of two types of Call signals: a continually persisting signal so long as the vehicle remains in the loop (the presence function); or a fixed length pulse generated when the vehicle is detected in the loop (the pulse function). Still other vehicle detectors are provided with selectable different sensitivity settings, which enable a technician to adjust the response of the vehicle detector when connected to the loop in order to accommodate a range of detection conditions.
In the past, all such selectable features have been implemented in vehicle detectors using manually settable switches, such as those found in a dual inline package (DIP switches). Because such switches provide only a fixed number of possible combinations, the number of selections available for each feature has been constrained by the use of DIP switches. While this does not necessarily pose a problem with some features, such as the pulse/presence selection or the end of green function (which require only one switch), the variety of choices available for the other features has been severely limited. In addition to this constraint, the use of mechanical switches creates a reliability problem due to the fact that switch contacts become corroded in the severe environment in which vehicle detectors are typically installed, and due further to the fact that the reliability of even new switches is not 100 percent. Even individual testing of each new switch obtained from a manufacturer does not always uncover defects in operation. This problem is compounded by the fact that vehicle detectors must be cost competitive in the marketplace, which creates a bias in favor of using the least expensive switch available.
In the past, vehicle detectors have been designed as either single channel or multiple channel detectors. A single channel detector is designed and configured to operate with only a single loop zone; while a multiple channel vehicle detector is designed and configured to operate with two or more independent loop zones. Multiple channel detectors are designed to be either scanning or non-scanning detectors. A scanning detector operates by sampling only one loop channel at a time, shutting down the active loop, sampling the next loop channel, shutting down that loop, etc. Scanning detectors are typically used in installations in which the probability of cross-talk between loop circuits is more than minimal. Cross talk results when physically adjacent loops are operating at, or near, the same frequency. Cross talk is minimized or eliminated by operating physically adjacent loops on different frequencies. Non-scanning vehicle detectors are configured and function to monitor each of the multiple loop zones simultaneously. Non-scanning detectors are typically used in installations in which there is a very low or no possibility of cross-talk between the multiple loop circuits, such as installations at which the loops are physically separated by a distance sufficient to ensure no overlapping or intercoupling between the electrical fields associated with the loops.
Vehicle detectors are typically installed in locations at which severe environmental conditions occur. For example, over a 24-hour period, the temperature of the loop environment in many locations can vary from well below freezing to well above 100.degree. F. Such temperature variations affect the operating frequencies of the loop oscillator circuit. In addition, humidity conditions vary widely over time and also affect the operating frequencies of the loop oscillator circuit. In order to provide adequate separation in the operating frequencies between physically adjacent loops, vehicle detectors have been provided with different selectable loop frequencies, using DIP switches or the like. The problems noted above with the use of such switches are equally applicable to this frequency select feature. In addition, this arrangement suffers from the disadvantage that those capacitors which are inserted into the oscillator circuit by closing the mechanical switches remain electrically connected to ground even when the oscillator circuit is shut down. An adjacent loop and oscillator circuit can still be adversely affected by such electrical connections.
During typical operation of a vehicle detector, momentary and long term power outages are experienced, and loops are electrically shorted or opened, either on a momentary basis or on a permanent basis. In order to provide some measure of the past operational condition of a given vehicle detector circuit, visible indicators (typically LEDs) and associated circuits have been used to indicate a past loop failure. Typically, the LED is connected to a flashing circuit, which in turn is controlled by a control unit within the vehicle detector, and which causes the visible indicator to flash if a loop failure occurred in the past. While useful to a technician inspecting a vehicle detector, this loop failure feature provides no statistical information, such as the number of failures since the last inspection, or the type of failures (i.e., open loop or shorted loop). In addition, when the detector is physically removed from the site and taken to a repair or replacement location, the illumination of the indicator stops and no further information exists regarding the past loop failure.
In view of the above, a need exists for a vehicle detector capable of improved performance over vehicle detectors of the known type.